Process for bleaching kraft pulp with oxygen in the presence of white liquor from a kraft recovery system

ABSTRACT

AN IMPROVED METHOD OF BLEACHING KRAFT PULP IN AN OXYGEN-BASED BLEACHING PROCESS IS DISCLOSED IN WHICH (A) WHITE LIQUOR FROM A KRAFT RECOVERY SYSTEM IS EMPLOYED AS A SOURCE OF ALKALI IN THE OXYGEN-BASED BLEACHING PROCESS WITHOUT ANY PRETREATMENT OF THE WHITE LIQUOR TO REMOVE SODIUM SULFIDE AND (B) THE EFFLUENT FROM THE OXYGENBASED BLEACHING PROCESS IS RECYCLED DIRECTLY TO THE KRAFT RECOVERY SYSTEM.

United States Patent 3,798,119 PROCESS FOR BLEACHING KRAFT PULP WITH OXYGEN IN THE PRESENCE OF WHITE LIQUOR FROM A KRAFT RECOVERY SYSTEM Rudra P. Singh, Ridley Park, Pa., assignor to Scott Paper Company, Philadelphia, Pa. No Drawing. Filed Apr. 24, 1972, Ser. No. 247,119

Int. Cl. D21c 9/10 US. Cl. 16265 Claims ABSTRACT OF THE DISCLOSURE An improved method of bleaching kraft pulp in an oxygen-based bleaching process is disclosed in which (a) white liquor from a kraft recovery system is employed as a source of alkali in the oxygen-based bleaching process Without any pretreatment of the white liquor to remove sodium sulfide and (b) the effluent from the oxygenbased bleaching process is recycled directly to the kraft recovery system.

BACKGROUND OF THE INVENTION Field of the invention Description of the prior art Wood is composed of two main partsa fibrous carbohydrates or cellulosic portion and a non-fibrous portion comprising phenolic-type compounds, this latter portion being commonly referred to as lignin.

In the kraft, or sulfate, pulping process, wood chips are treated, in a digester, with a solution of sodium hydroxide and sodium sulfide which attack the non-firous (lignin) portion of the wood so the individual fibers can be separated. The resulting pulp is then transferred to a brown stock washing system wherein chemicals are removed by washing with water. The eflluent from this washing operation plus the spent cooking liquor from the digester is commonly referred to as black liquor. The black liquor contains practically all the alkali originally added, plus about half the original weight of the wood .chips. Weak black liquor is either recycled to the digester where its acts as a diluent for the fresh liquor, or it is treated as follows in a kraft recovery system:

The weak black liquor (at approximately 15-20% solids) is treated in a series of evaporators, commonly referred to as a multiple effect evaporator, and is discharged to a direct contact evaporator where further water is evaporated until the solids content is approximately 65-70%. The evaporator liquor is then screened and passed through a recovery unit where (a) make-up chemical or salt cake (sodium sulfate) is added; (b) the remaining water is removed, (c) sodium salts are recovered mostly as sodium carbonate and sodium sulfide; and ((1) heat is obtained by burning the organic compounds contained in the liquor.

The molten ash, or smelt, from the recovery unit, which consists of sodium carbonate and sodium sulfide, is dissolved in water to form green liquor. The green color is dueto iron impurities which, along with silica and other suspended solids, are removed by treatment .with

coagulating agents and settlement in a clarifier. The clarification step is followed by a causticizing step in which the green liquor is treated with lime (calcium oxide) to convert the sodium carbonate to sodium hydroxide, the sodium sulfide remaining unchanged. The sludge from the lime treatment is removed, and the resulting solution is referred to as white liquor which can be reused as cooking liquor in the digester. The sludge is washed in a lime mud washer and the filtrate used to dissolve additional smelt from the recovery unit. The lime mud is treated in a kiln to yield reburned lime. During the burning operation some calcium oxide and sulfur compounds are collected in the kiln stack and removed by a scrubber in said stack.

Unbleached kraft, or sulfate, pu'lps are generally bleached to an acceptable brightness by a multistage process employing chlorine or chlorine-containing compounds such as calcium or sodium hypochlorite and chlorine dioxide. Although chlorine and chlorine-containing compounds have proven to be effective bleaching agents, these compounds are difiicult to handle, introduce the problem of corrosion to the paper-making equipment, and render the efiluents from the bleach plant incapable of being recovered and recycled. In addition waste liquors and wash water from the bleach plant incorporating such compounds can cause a serious pollution problem. The stream pollution problems associated with eflluents from chlorine and/or chlorine-containing bleaching processes are well recognized, and it has been suggested that these effluents be concentrated and burned in a pulping liquor recovery system such as the one described above. However, this method of disposing of such efiluents has several major disadvantages which have discouraged the use of such a recovery and disposal system for chlorine-containing bleach plant eflluent. The chloride content of the eflluent introduces the problems of corrosion, solid buildup, volatilization of toxic compounds, etc., in the recovery system.

Alternative bleaching methods such as those employing oxygen have been suggested for use in bleaching wood pulp. See, in this regard, US. Pat. 3,024,158 which discloses the use of oxygen to bleach wood pulp and prevent brightness reversion in the bleached pulp; and US. Pat. 3,384,533 relating to catalysts useful in bleaching wood pulp with oxygen in an alkaline medium. Other suggestions that kraft pulp can be bleached with a combination of oxygen and alkali are found, for example, in the article by Hiisvirta appearing in 53 Paperi ja Puu, No. 7, pages 439-443 (July 1970) and in the article by Rowlandson in Paper Trade Journal, Dec. 21, 1970, page 38. The oxygen bleaching of wood pulp is also generally disclosed in South African patent application No. 68/ 3771 filed June 13, 1968 by the South African Pulp and Paper Industries, Limited et al. In all of these references wood pulp is treated with oxygen in an alkaline environment at an elevated temperature and an elevated pressure to bleach the plup.

It also been suggested to treat the efiluent from such an oxygen bleaching operation by introducing it into a chemical recovery process. However, in all of the prior art, it was necessary to treat the efiluent from the bleaching operation prior to its introduction into the recovery system to remove at least part of the sodium hydroxide contained in the efiluent. To introduce all of the sodium hydroxide into, for example, a conventional kraft recovery system which also contained a high percentage of sodium hydroxide from the pulping operation would result in an imbalance in the recovery system which could not be tolerated in previous processes.

Pre-treatments of the wood pulp with acids such 'as sulfuric acid and sulfur dioxide prior tothe oxygen bleaching step have also been suggested as a means for decreasing the degradation of the wood pulp which might otherwise occur during the bleaching operation. However, these acid pre-treatments were always followed by a washing step prior to the bleaching operation since it appeared that to introduce acid into the bleaching opera tion would result in a decrease in the efliciency of said operation due to a neutralization of some of the alkali required for bleaching.

SUMMARY OF THE INVENTION In accordance with the present invention kraft pulp is bleached in an improved oxygen-based bleaching process wherein the improvements comprise (a) employing untreated white liquor from a kraft recovery system as a source of alkali in the oxygen-based bleaching process and (b) recycling the eflluent from the bleaching process directly to the kraft recovery system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the present invention involves the combination of a kraft recovery system and an oxygenbased bleaching process. Pulp to be treated in accordance with the present invention is prepared in a conventional kraft pulping operation as is well-known to those skilled in the art. The effluent from this pulping operation is then introduced into a kraft recovery system, such as that described above. The resulting pulp is finally bleached to a high brightness by an oxygen-based bleaching process as is also well-known to those skilled in the art. What is done in accordance with the present invention is to (la) employ untreated white liquor from the kraft recovery system as a source of alkali in the oxygen-based bleaching process and (b) to introduce the efiluent from the oxygen-based bleaching process directly into the kraft recovery system without any pre-treatment of said effluent.

In accordance with the present invention it has been found that untreated white liquor from the kraft recovery system can be employed as a source of alkali in the oxygen bleaching process without having any detrimental eifect on the brightness or other properties of the resulting pulps. Asmentioned above, the white liquor contains, in addition to the desired sodium hydroxide, a significant amount of sodium sulfide which is known to be a strong reducing agent. It had been expected that the introduction of a reducing agent into the bleaching process would decrease the efficienc of the oxidation reaction occurring therein. However, this has not been found to be the case. To the contrary, it has been found that white liquor having a sulfidity of from about 20% to about 40% may be employed in the bleaching operation without, in 'any way, degrading the properties of the resulting pulp.

In carrying out the bleaching operation a conventional oxygen-based bleaching process is employed. However, in accordance with the present invention it is preferred to carry out this process utilizing pulp at a consistency of from about 20% to about 40%, an oxygen pressure of from about 80 to about 150 p.s.i., and 'an elevated temperature of from about 80 C. to about 160 C. fora time of from about 20 to about 90 minutes. Optimum results have been achieved when the bleaching operation is carried out at an alkaline pH of from about 8 to 13.

In accordance with the present invention, it has been discovered that the required alkaline pH may be achieved by treating the pulp with the white liquor obtained from a kraft recovery system without any pre-treatment of the white liquor to remove the sulfur contained therein. Although the sulfidity of white liquor is generally in the range of from about 20 to about 25%, it has been found that satisfactory results can be achieved when a white liquor having a sulfidity of up to about 40%, such as those commonly encountered in commercial operations,

is employed. The amount of White liquor employed in treating the pulp prior to its treatment with oxygen in the bleaching process can be varied over a wide range depending upon the type of pulp employed and the properties desired in the final pulp. For a conventional hardwood pulp satisfactory results have been achieved when the amount of white liquor employed is sufiicient to add from about 1% to about 8% active alkali based on the dry weight of the pulp. To produce a pulp useful in the preparation of sheets having optimum physical prop erties, it is especially preferred to employ an amount of white liquor such that from about 1% to about 4% active alkali is "added to the pulp.

The oxygen-based bleaching process may be utilized either alone or in combination with other treatments conventionally employed in the bleaching of wood pulp. It has also been found, in accordance with the present invention, that efiiuent from the above-described, oxygenbased bleaching operation can be introduced directly into the kraft recovery system. Thus, no pre-treatment is required to remove excess alkali from this efiluent prior to its recycling. It is especially preferred to introduce the effluent into the recovery system by utilizing it in the lime kiln scrubber or brown stock washer so that the solids contained therein are removed, incinerated and causticized in the normal operation of that system.

One advantage of the present process is that it results in a minimum amount of fresh water being required and in a significant reduction in the stream and air pollution associated with the previously employed pulping and bleaching operations.

In order to describe the present invention so that it may be more clearly understood, the following examples are set forth. These examples are set forth primarily for the purpose of illustration, and any specific enumeration of detail contained therein should not be interpreted as a limitation on the concept of this invention.

The following standard procedures were employed in carrying out the examples:

Handsheets were prepared by the procedure described in TAPPI Standard T-28 lm for optical tests and in accordance with TAPPI Standard T-205m for physical tests.

Brightness of the sheets was measured on a General 'Electric photometer in accordance with TAPPI Standard T-217m and is expressed in terms of GE. units.

Tensile strength of the sheets was measured in accord ance with TAPPI Standard T-220m60 and is expressed in terms of meters.

The tear value of the handsheets was also measured in accordance with TAPPI Standard T-220 using a junior Elmendorf tear tester.

The viscosity of the pulp was measured in accordance with TAPPI Standard T-230 and is reported in terms of centipoise.

EXAMPLE I Poplar hardwood chips were reduced to a pulp by the kraft process. Handsheets prepared from a sample of the pulp had a brightness of 36.0.

A sample of the pulp, at a consistency of approximately 26%, was treated with a solution of magnesium carbonate so that about 1% by weight, based on the weight of the fibers, was added. The fibers were then treated with white liquor from a kraft recovery porcess so that 4.5% by weight, based on the weight of the fiber, of the white liquor was added. The treated sample was placed in a Parr bomb which was charged with oxygen until the pressure rose to approximately p.s.i. The Parr bomb was maintained at this pressure and heated to approximately 100 C. for a period of about 30 minutes. The pulp was then removed from the bomb and washed thoroughly with water and its pH adjusted to about 4.0

The effluent from the bleaching step is collected and introduced directly into the chemical recovery system of the kraft pulping process.

A second sample of the pulp was treated as above except that the white liquor was replaced by an equivalent amount of an aqueous solution of sodium hydroxide. Handsheets prepared from pulp bleached in this manner had a brightness of about 52.3.

'EXAMPLE II Maple hardwood chips were pulped by the kraft process. Handsheets prepared from a sample of the pulp had a brightness of 37.5.

Four samples of the pulp were treated with white liquor having a sulfidity of about 25% so that various percentages of active alkali based on the dry weight of the pulp were added. The treated samples, at a consistency of about 25%, Were placed in a Par-r bomb and charged with oxygen until the pressure increased to approximately 150 p.s.i. The Parr bomb was then heated to about 125 C. and maintained at a temperature of from about 125 C. to about 165 C. for 90 minutes, during which time the pressure rose to about 250 p.s.i. The pulp was then removed, washed with water and the pH adjusted to about 4.0 by the addition of sulfurous acid. The resulting pulp was tested for optical and physical properties.

These results are summarized in the following table:

Percent active alkali Bright- Tensile Tear Sample added Viscosity ness strength value A 1. 00 12. 4 67. 2 5, 575 39 B 4. 65 4. 6 76.0 4, 825 26 C 8. 75 2. 2 80. 7 3, 058 8 D 15. 00 2. 4 83. 0 548 8 Bright- Tensile Tear V1scosity ness strength value EXAMPLE III Four additional samples of the pulp prepared in Example II were treated with white liquor having a sulfidity of about 40% so that various percentages of active 6 alkali based on the dry weight of the pulp were added. The pulp was treated and tested as in Example II. The results are summarized in the following table:

Percent active alk Bright- Tensile Tear added Viscosity ness strength value What is claimed is:

1. In a method of bleaching kraft pulp in an oxygenbased bleaching process, the improvement which com prises, employing white liquor from a kraft recovery system as a source of alkali in the oxygen-based bleaching process in an amount suflicient to introduce from about 1% to about 8%, by Weight, active alkali based upon the weight of pulp, without any pretreatment of the white liquor to remove sodium sulfide, to obtain a bleached pulp without any significant degrading of the properties of the bleached pulp.

2. A method as claimed in claim 1, in which the sulfidity of the white liquor employed is equal to from about 20% to about 40%.

3. A method as claimed in claim 1, in which the amount of white liquor employed is sufiicient to introduce from about 1% to about 4% by weight active alkali based on the weight of the pulp.

4. A method as claimed in claim 1, in which effluent from the oxygen-based bleaching process is introduced directly into the kraft recovery system.

5. A method as claimed in claim 4, in which the efiluent is introduced into the kraft recovery system at the brown stock washer.

References Cited UNITED STATES PATENTS 2,811,518 10/1957 Mitchell et al 162-65 X 3,311,530 3/1967 Justus et a1 16265 X 3,654,070 4/ 1972 Pradt et al. 16265 X 3,366,534 1/1968 Rapson 16230 3,453,174 7/1969 Rapson 162--30 3,525,665 8/ 1970 Gessner 162--65 X 3,384,533 5/1968 Robert et a1. 162-65 3,701,712 10/1972 Samuelson et al 16265 OTHER REFERENCES Rydholm: Pulping Processes, 1965, pp. 731 and 732. (GP

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner 

